Malaria is a disease caused by various species of hemosporidian blood parasites of the genus Plasmodium (including P. falciparium, P. vivax, P. ovale, and P. malariae). The human disease process begins with the bite of an infected female Anopheline mosquito. Plasmodium sp. sporozoites released from the salivary glands of the mosquito enter the bloodstream and quickly (within about 30 minutes) invade liver cells (hepatocytes). The liver-stage parasites differentiate and undergo asexual multiplication resulting in tens of thousands of merozoites which burst from the hepatoctye. Merozoites then invade red blood cells (erythrocytes) where they undergo an additional round of multiplication. The clinical signs of malaria, fever and chills, are associated with the synchronous rupture of infected erythrocytes.
Malaria is a worldwide public health problem. Up to 3 million people die of malaria each year, and the total number of people infected with malaria worldwide approaches half a billion. The impact of this disease, in terms of suffering and economics, is huge. In the past, the inexpensive and orally administered antimalarial drug, chloroquine (CQ), was the “gold standard” treatment.
During its blood stage, the malaria parasite metabolizes (in its digestive vacuole (DV)) hemoglobin present in erythrocytes. A by-product of this metabolic process is heme, which would be toxic to the parasite except for heme detoxification mechanisms developed by the organism. CQ is believed to inhibit heme detoxification in the DV by binding to heme and/or to hemozoin (Ginsburg et al., Parasitol. Today, 15:357, 1999; Chong and Sullivan, Biochem. Pharmacol., 66(11):2201-2212, 2003; Leed et al., Biochem., 41(32):10245-10255, 2002; Egan et al., Biochem., 40(1):204-213, 2001; Raynes, Int. J. Parasitol., 29(3):367-379, 1999; Egan et al., J. Med. Chem., 43(2):283-291, 2000). This binding, in part, is thought to drive the thermodynamics that concentrates CQ in the parasite DV. Chloroquine-dependent heme accumulation in the DV may inhibit or kill the parasites.
Unfortunately, certain Plasmodium sp. strains have evolved resistance to CQ. In fact, the spread of chloroquine-resistant (CQR) Plasmodium sp. parasites has rendered CQ almost useless for malaria treatment. In addition, Plasmodium sp. resistance to other antimalarial drugs, such as artemisinin and its derivatives, has been reported (Xiao et al., Parasitol. Res., 92(3):215-219, 2004). These are particularly devastating problems in many impoverished parts of the world where such drugs are most needed (Krogstad, Epidemiol. Rev., 18(1):77-89, 1996).
Plasmodium sp. CQR is correlated with mutations in the parasite's DV membrane transporter protein (PfCRT). PfCRT is thought to enhance CQ export from the DV (Zhang et al., Biochem., 43(26):8290-8296, 2004; Bennett et al., Mol. Biochem. Parasitol., 133(1):99-114, 2004; Martin and Kirk, Mol. Biol. Evol., 21(10):1938-1949, 2004). A particularly well-studied PfCRT mutation is K76T, which is correlated with CQR (Martin and Kirk, Mol. Biol. Evol., 21(10):1938-1949, 2004; Johnson et al., Mol. Cell, 15(6):867-877, 2004; Durrand et al., Mol. Biochem. Parasitol., 136(2):273-285, 2004; Ranjit et al., Trop. Med. Int. Health, 9(8):857-861, 2004; Durand et al., Antimicrob. Agents Chemother., 46(8):2684-2686, 2002; Durand et al., Mol. Biochem. Parasitol., 114(1):95-102, 2001; Cooper et al., Mol. Pharmacol., 61(1):35-42, 2002; Djimde et al., N. Engl. J. Med., 344(4):257-263, 2001).
Alternative therapies for treatment of chloroquine-resistant (CQR) malaria parasites have been developed, including combination therapies (Kumar et al., Curr. Med. Chem., 10(13):1137-1150, 2003). However, none of these therapies meet CQ's simplicity of use and low cost.
One approach to treatment of CQR parasites involves the use of chemicals known as “reversal agents.” Reversal agents are chemicals that have been found to quash CQ resistance (Krogstad et al., Science, 238(4831):1283-1285, 1987; Martin et al., Science, 235(4791):899-901, 1987; Ryall, Parasitol. Today, 3(8):256, 1987); thus, making CQR strains sensitive again to CQ. CQR reversal agents do not appear to have independent therapeutic value against Plasmodium sp. because the therapeutically useful dose for CQ applied to a CQS strain in the presence of a reversal agent is nearly the same as in the absence of the reversal agent.
At least some reversal agents are believed to act by inhibiting CQ export from the DV (Krogstad et al., Science, 238(4831):1283-1285, 1987; Martin et al., Science, 235(4791):899-901, 1987). However, despite their effectiveness against CQR Plasmodium sp. strains in vitro, therapeutically effective doses of many reversal agents would have to be quite high (with associated side effects) to reverse CQR in vivo (van Schalkwyk et al., Antimicrob. Agents Chemother., 45(11):3171-3174, 2001; Millet et al., Antimicrob. Agents Chemother., 48(7):2753-2756, 2004).
New compositions and methods for the treatment of malaria, particularly for the treatment of CQR malaria parasites, are needed.